Together with complex ligands used in excess, the rhodium complex compounds form inter alia, a catalyst system which--as described in the DE-PS 26 27 354--is used for the hydroformylation of olefins. Under the reaction conditions, the catalyst system is formed from rhodium and excess water-soluble organic phosphines. Its water-solubility is due to the presence of sulfonic acid groups which are in the organic phosphines. The phosphorus ligands are preferably alkali sulfonate, ammonium sulfonate, or alkaline earth sulfonate.
With prolonged use of the catalyst system, the selectivity of the reaction decreases. This loss of selectivity is due to (1) the effects of catalyst poisons, such as iron carbonyl, which can form through the action of carbon monoxide on the wall of the reactor, (2) the formation of higher-boiling condensation products from the aldehydes, and (3) the decrease in the sulfonated phosphine due to oxidation to phosphine oxides or decomposition to aromatic sulfonic acids. In addition, phosphine sulfides also form from the phosphines and the sulfur compounds contained in the synthesis gas as well as from reduction of sulfonic acid groups.
As neither phosphines oxides, phosphine sulfides, nor aromatic sulfonic acids are desirable in the hydroformyation catalyst, the spent catalyst solution must be replaced. For economic reason, it is necessary to separate and recover the rhodium from this catalyst solution. It is of paramount importance that the rhodium be recovered as completely as possible in a form which permits reuse as a catalyst component.
DE-OS 36 26 536 teaches a process for recovering rhodium from aqueous solutions containing rhodium complex compounds. An excess, based on the rhodium, of a water-soluble salt of a carboxylic acid with 7 to 22 carbon atoms is added to the solution, the solution is then treated at 50.degree. to 200.degree. C. with an oxidant, and the precipitated rhodium is separated. In this manner, approximately 90 to 95% of the rhodium present in the solution can be recovered. Oxygen, air, or hydrogen peroxide can all be used as oxidants. However, it has been shown that the percentage recovered depends quite substantially on the type of solution. Unforeseen changes, which presumably occur due to ageing and the reaction conditions under which the solution is used as a catalyst, impede trouble-free rhodium recovery.
The rhodium which is not recovered can, however, be separated only at great cost; for example, by oxidation at high temperatures and pressures or thermal decomposition, if necessary with the aid of a support material such as activated carbon, onto which the rhodium is precipitated. With this process, the rhodium is recovered either as a metal or in the form of compounds which can no longer be used directly as catalyst components, in particular for hydroformylation.